The asynchronous transfer mode (ATM) protocol was designed for communication over point-to-point physical links between network elements and network terminals. Such an interface is known as the user-network interface or UNI. However, many access network topologies, including hybrid fiber coax and fiber to the curb, are based on a point-to-multipoint topology, where a shared medium link exists between a master network element and multiple slave network terminals. Such an interface is known as the multi UNI or MUNI interface. In this situation, the ATM protocol will require enhancements to support the point-to-multipoint network topology from an addressing point of view and from traffic management point of view.
There have been many proposals to deal with the addressing issue of handling ATM connections in a point-to-multipoint shared medium network topology. A first proposal is to prepend each ATM cell with a physical layer overhead containing addressing information to identify which of the multiple devices connected to an asynchronous transfer mode edge switch is the destination/source of the cell in question. However, this technique requires the development of new physical layer devices and framers to extract and generate the prepended overhead bytes from and to the standard fifty-three byte ATM cell. Moreover, this technique adds a per cell overhead that reduces the efficiency of the transmission link.
Another technique takes bits in the ATM cell header and changes them from their intended use, in a standard user to network interface or in a standard network to network interface cell protocol, in order to specify a specific terminal address on the shared medium. This technique would require the masking of bits in one direction stream and insertion of bits in another direction by an adapter device attached to the slave terminal of the master ATM edge switch. Consequently, if the virtual path identifier (VPI) and/or virtual circuit identifier (VCI) fields of the ATM cell header are used for that purpose, without a formalized set of rules for usage, then many adverse complications will emerge in the ATM signaling/meta-signaling and in ATM operations, administration, and maintenance functions due to the fact that the ATM standard has defined some well known VPI/VCI values to carry meta-signaling and OAM cells, and any non-standard manipulation VPI/VCI bits will conflict with these well known values. If the entire VPI field is used for that purpose, then the disadvantages to this technique is that a terminal can only have one virtual path (VP) and the virtual path is not seen similarly on both sides of the links, since it is being modified by an active device between the two endpoints. Consequently, the header error check field in each ATM cell is to be recalculated. No proposals have been made public that address the issues related to traffic management from a point-to-multipoint ATM link. Therefore, it is desirable to have a point-to-multipoint addressing technique that avoids using a prepended physical layer header and avoids single virtual path bit robbing found in conventional point-to-multipoint techniques.